1. Field of the Invention
The present invention relates to a conductive material for a connecting part such as a connector terminal, bus bar, and so forth, used in electrical wiring mainly for automobiles, consumer equipment, and the like, and in particular, to a conductive material for a connecting part, of which reliability of electrical connection in applications as well as reduction in friction and wear upon insertion.
2. Description of Related Art
For the conductive material for a connecting part such as the connector terminal, bus bar, and so forth, used in electrical wiring for automobiles, consumer equipment, and the like, Cu or a Cu-alloy, with Sn plating applied thereto, (including an Sn-alloy plating such as solder plating and so forth) is used, except the case of an important electrical circuit requiring high reliability of electrical connection, against a low-level signal voltage and current. Sn plating has been in widespread use because it is lower in cost in comparison with Au plating, and any other means for surface treatment. Among others, Sn plating containing no Pb from a standpoint of coping with recent regulations against material causing environmental impacts, and particularly, reflow Sn coating, and hot dip Sn coating, on which there have hardly been reported a case of short circuit trouble due to occurrence of whiskers, are now in the mainstream.
As a leap forward development has recently been made in electronics rapid progress has been seen in higher use of electrical equipment in, for example, automobiles, in an attempt to pursue safety, environmental friendliness, and driving comfort. As a result, there occurs an increase in the number of circuits, weight thereof, and so forth, leading to an increase in space occupied, and energy consumption, so that there arise requirements for a conductive material for a connecting part capable of providing a satisfactory performance required of the connecting part such as a terminal, bus bar and so forth even in the case of a multi-way connector, further reduction in size as well as weight, and a connecting part mounted in an engine room.
The Sn plating is applied to the conductive material for the connecting part mainly for the purpose of providing a surface thereof with corrosion resistance while obtaining a low contact resistance at electrical contacts and junctions and securing solderability when the conductive materials for the connecting parts are joined together by soldering. An Sn plating is a very soft conductive film, and an oxidized surface film thereof is prone to fracture. Accordingly, in the case of a fitting type terminal made up of a male form terminal in combination with a female form terminal, electrical contacts, such as indents, ribs, and so forth, tend to easily form gas tight contact due to adhesion occurring between the plating layers to be thereby rendered suitable for obtaining a low contact resistance. Further, in order to maintain the low contact resistance in applications, an Sn plating is preferably larger in thickness, and it is important to increase a contact pressure at which the electrical contacts are pressed against each other.
However, if the Sn plating is rendered larger in thickness, and the contact pressure at which the electrical contacts are pressed against each other is increased, this will cause an increase in a contact area between the Sn plating layers, and an increase in an adhesion force therebetween, so that there occurs an increase in a deformation resistance due to the Sn plating layer being turned up at the time of insertion of the terminal, and an increase in a shearing resistance for shearing adhesion, thereby resulting in an increase in an insertion force. A fitting type connecting part large in insertion force will cause poor efficiency of assembling work, and deterioration in electrical connection due to wrong fitting. Accordingly, there is a demand for terminals low in insertion force so that the total insertion force thereof does not become greater than that in the past even if the number of poles is increased.
Further, in the case of a small-sized Sn plated terminal, and so forth, with a reduced contact pressure under which electrical contacts are pressed against each other, for the purpose of reducing the insertion force thereof, and wear occurring thereto at the time of insertion of the terminal, and pull-out thereof, not only it becomes difficult to maintain a low contact resistance in subsequent applications but also the electrical contacts are caused to undergo slight sliding due to vibration, thermal expansion/contraction, and so forth, during applications, so that the small-sized Sn plating terminal will be susceptible to occurrence of a slight-sliding wear phenomenon causing an abnormal increase in contact resistance. It is presumed that the slight-sliding wear phenomenon is induced by wear occurring to the Sn plating layers at electrical contacts, due to the slight-sliding, and by deposition of a large amount of resultant Sn oxide between the electrical contacts, due to repetition of the slight-sliding. For reasons described as above, there is a demand for a terminal low in the insertion force, excellent in resistance to wear upon insertion thereof, and pull-out thereof as well as resistance to wear due to the slight-sliding so as to be capable of maintaining a low contact resistance in spite of an increase in the number of actions for the insertion and pull-out, and the slight-sliding occurring to the Sn plating layers at electrical contacts.
More and more connector terminals, bus bars and other automotive parts are mounted inside engine rooms at high temperatures to meet the demands for space savings, and their calorific values are also increased by the Joule heat because of an increase in the amount of current used. For these reasons, such parts are increasingly used in more harsh thermal environments. In Sn plated terminals, bus bars and like parts in which the thickness of a Sn covering layer formed is small for reducing insertion force and for other purposes, a thick layer of constituent elements other than Sn which causes thermal diffusion from materials and undercoat plating is formed as oxide by thermal influence, and contact resistance is increased since their electric resistances are high, whereby reliability of electrical connection is lost. In addition, Sn plated terminals, bus bars and the like may experience peeling of Sn plating by the thermal diffusion phenomenon, depending on the type of the base material made of Cu alloy, leading to a loss of reliability of electrical connection. Accordingly, connector terminals, bus bars and the like having small insertion force and excellent heat resistance in which low contact resistance can be maintained and peeling of Sn plating does not occur even in a harsh thermal environment are required.
Connector terminals, bus bars and like parts for automobiles are sometimes used in a harshly corrosive environment such as an exhaust gas atmosphere. In Sn plated terminals, bus bars and like parts in which the thickness of a Sn covering layer formed is made small for reducing insertion force and for other purposes, not only the Sn covering layer but also an undercoat covering layer and material constituent elements are thickly formed on the entire surface as a corrosion product by being retained in a corrosive environment for a long period of time. Since their electric resistances are high, their contact resistances are increased and therefore their reliability of electrical connection is lost. Accordingly, connector terminals, bus bars and like parts having low insertion force and excellent corrosion resistance in which low contact resistance can be maintained even when they are retained in a corrosive environment for a long period of time are required.
In addition, for connector terminals which are connected by soldering, joining using a solder containing no Pb (Pb-free solder joining) is increasingly used from a standpoint of coping with recent regulations against materials causing environmental impacts. The Pb-free solder has a higher melting point, lower solder wettability, and higher ability to dissolve connector terminals and the like than Sn—Pb-based solders which have been used in known solder joining. Therefore, ensuring joining reliability are sometimes difficult in conventional Sn plated terminals and the like. To ensure reliability in soldering joining with printed circuit boards and the like, it is important to form a plated layer as an undercoat or a thick Sn plated layer, but insertion force is increased at a mating portion of another connector terminal. Therefore, a connector terminal having low insertion force and excellent solderability is required.
Japanese Unexamined Patent Application Publication (JP-A) No. 2004-68026, JP-A-2003-151668, JP-A-2002-298963, JP-A-2002-226982, JP-A-11-135226 and JP-A-10-60666 describe materials for a fitting type terminal, wherein an Ni plating layer as an undercoat is formed on the surface of a base material composed of Cu or a Cu-alloy, and after forming a Cu plating layer, and an Sn plating layer in that order on the top of the Ni undercoat plating layer, a reflow process is applied thereto, thereby forming a Cu—Sn alloy covering layer. According to the description in these documents, the Cu—Sn alloy covering layer formed by the reflow process is harder as compared with the Ni plating layer, and the Cu plating layer, and owing to presence of the Cu—Sn alloy covering layer as an undercoat layer of the Sn covering layer remaining on the uppermost surface of the material, it is possible to decrease the insertion force of the terminal. Further, a low contact resistance can be maintained by the Sn covering layer present on the surface.
As the thickness of the Sn covering layer on the surface of the terminal becomes smaller, the insertion force of the terminal with the Cu—Sn alloy covering layer formed as the undercoat of the Sn covering layer is lowered. Further, the insertion force of the terminal with the Cu—Sn alloy covering layer formed on the surface thereof undergoes a further decrease. On the other hand, if the Sn covering layer becomes smaller in thickness, there will arise a problem that there occurs an increase in contact resistance of a terminal in the case where the terminal is held in a high-temperature environment reaching 150° C., for example, in an engine room of an automobile for many hours. Further, if the Sn covering layer is small in thickness, both corrosion resistance and solderability undergo deterioration. In addition, the Sn covering layer is susceptible to occurrence of the slight-sliding wear phenomenon. Thus, there have not been obtained as yet satisfactory properties required of conductive materials for connecting parts, such as a low insertion force, maintenance of a low contact resistance even in a corrosive environment or a vibrating environment after frequent insertions and pull-out of the terminal, and after the terminal being held in an high-temperature environment for many hours, excellent solderability and so forth, so that further improvements are required.
Meanwhile, JP-A-2006-77307 describes a conductive material for a connector part including a base material made up of a Cu strip, a Ni covering layer (including 0 μm), a Cu covering layer (including 0 μm), a Cu—Sn alloy covering layer and an Sn covering layer formed over a surface of the base material in the order stated, wherein a ratio of an exposed area of the Cu—Sn alloy covering layer to a surface of the material is 3 to 75% (desirably, an average surface material exposure interval in at least one direction is in a range of 0.01 to 0.5 mm), the Cu—Sn alloy covering layer having an average thickness is 0.1 to 3.0 μm and an amount of Cu contained of 20 to 70 at %, and the Sn covering layer having an average thickness of 0.2 to 5.0 μm. In the invention of JP-A-2006-77307, used is a base material in which an arithmetic mean roughness Ra of a surface of the base material, in at least one direction, is not less than 0.15 μm, and the arithmetic mean roughness Ra thereof, in all directions, is not more than 4.0 μm (desirably, an average interval between projections and depressions, in at least one direction, is 0.01 to 0.5 mm), and a Cu plating layer and an Sn plating layer are formed on the surface of the base material, or an Ni plating layer, a Cu plating layer and an Sn plating layer are formed, and then a reflow process is carried out.
JP-A-2006-183068 describes a conductive material for a connecting part, in which an Ni covering layer (including 0 μm), a Cu covering layer (including 0 μm), a Cu—Sn alloy covering layer and an Sn covering layer are formed in the order stated on the surface of a base material made up of a Cu strip, a ratio of an exposed area of the Cu—Sn alloy covering layer to a surface of the material is 3 to 75% (desirably, an average surface material exposure interval in at least one direction is in a range of 0.01 to 0.5 mm), an average thickness is 0.2 to 3.0 μm, an amount of Cu contained is 20 to 70 at %, the Sn covering layer has an average thickness of 0.2 to 5.0 μm, the surface of the material has an arithmetic mean roughness Ra of a surface of the base material, in at least one direction, is not less than 0.15 μm, and an arithmetic mean roughness Ra in all directions is 3.0 μm or less. In the invention of JP-A-2006-183068, used is a base material having an arithmetic mean roughness Ra in at least one direction of not less than 0.3 μm and an arithmetic mean roughness Ra thereof, in all directions, of not more than 4.0 μm (desirably, an average interval Sm between projections and depressions, in at least one direction of 0.01 to 0.5 mm), in which a Cu plating layer and an Sn plating layer are formed on the surface of the base material, or an Ni plating layer, a Cu plating layer and an Sn plating layer are formed, and then a reflow process is carried out.
The conductive materials for a connecting part described in JP-A-2006-77307 and JP-A-2006-183068, especially for use in the fitting type terminal, are capable of checking friction coefficient to a low level, and therefore an insertion force upon fitting a male terminal into a female terminal is low in the case where it is used for a multi-way connector, for example, in an automobile, so that assembling work can be efficiently carried out. Further, even after the material is held in a high-temperature environment for many hours, and in a corrosive environment, reliability of electrical connection (low contact resistance) can be maintained. In addition, in JP-A-2006-183068, it is possible to maintain reliability of electrical connection (low contact resistance) even in a vibrating environment. Furthermore, it can maintain excellent reliability of the electrical connection even when disposed in a spot for application at a very high temperature such as an engine room and the like.